Alternating current (AC) motors are used in a variety of applications. To power an AC motor, a current is typically supplied to the AC motor via a direct current (DC) bus voltage source, such as a battery. The current flows through windings in a stator or a rotor of the AC motor to produce a magnetic field. The current flow creates a torque within the AC motor causing the rotor to rotate and to thereby produce mechanical power. To reduce unwanted torque pulsations in the AC motor resulting from harmonics which may be present in the AC motor during current input, a voltage source inverter is used to transform the current from the DC bus voltage source into a sinusoidally-shaped motor phase current.
In some cases, the AC motor may be a permanent magnet (PM) machine. Typically, as the speed of the PM machine increases, the amount of current supplied to the PM machine increases. However, if the PM machine operates above a predetermined speed, a phase voltage within the PM machine may exceed a supplied bus voltage from the DC bus voltage, which may create a machine flux (also known as a back electromotive force (EMF)). Generally, a back EMF may undesirably reduce operational efficiency of the PM machine.
To retain control of the PM machine, the back EMF may be reduced using field-weakening. In this regard, a demagnetizing current typically is applied to the PM machine to reduce the magnet or total flux of the PM machine. Although this field-weakening technique operates sufficiently, it may be improved. In particular, additional components may be included in the PM machine in order to supply the demagnetizing current, which may undesirably increase weight and size of the PM machine. Moreover, supplying the demagnetizing current to the PM machine may direct energy resources to non-torque producing currents. Additionally, the rotational speed of the PM machine may be limited by the amount of demagnetizing current that may be supplied to counteract the back EMF.
Accordingly, it is desirable to have a PM machine that may reduce back EMF with minimal additional components. In addition, it is desirable for the improved PM machine to be capable of operating at rotational speeds that are greater than speeds at which conventional PM machines operate. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.